Due to the EOR need of injecting weak alkaline water in Linnan oilfield, developing surfactant system under the condition of weak alkaline water was of great significance. The production water in Linnan oilfield was neutral water with pH value of 6.89 and salinity of 44118 mg/L, while the treated water belonged to weak alkaline water with pH value of 7.88 and salinity of 39116 mg/L. By testing the interfacial tension between different types of surfactants and crude oil, the compound system CDS6 composed of fatty alcohol ether sulfonate (CEOS6) and amide-type nonionic surfactant (CDA01) was selected. The effect of solution medium on interfacial tension and the optimized slug injection and concentration of surfactant were also investigated. The experiment results showed that the constituent of optimum compound system CDS6 was obtained as follows: 0.1% CEOS6+0.2% CDA01. The interfacial tension of composite surfactant system in neutral water and weak alkaline water with crude oil was 5.84×10^-3 and 1.70×10^-3 mN/m, respectively. Weak alkaline water had synergistic effect with CDS6 in decreasing interfacial tension. The simulation flooding test results showed that the oil displacement efficiency of 0.3% CDS6 in weak alkaline water was higher than that in neutral water. The oil displacement efficiency of 0.3% CDS6 weak alkaline solution at the slug injection of 0.4 PV was 57.33% which was improved by 10.66% compared with that of water flooding.